Since a working machine in the form of a loading/unloading vehicle having booms supporting a load carrier, for example a bucket or a fork carried thereon such as a wheel loader, shovel loader or the like has advantageous features that it is designed and constructed in smaller dimensions, it can turn with a small radius and it can be purchased at an inexpensive cost, it has been widely utilized in many field sites of civil engineering works.
As shown in FIG. 9, this kind of loading/unloading vehicle is so constructed that booms 1 are vertically turned by means of a boom cylinder 3 (rising of the booms 1 being referred to as "lift") and a bucket 2 is turned to the tilt side (representing turning movement of the bucket to the vehicle body side (excavating side)) or to the dump side (representing reverse operation to the tilting operation, i.e., turning movement of the bucket to the gravel dump side). Thus, as the booms 1 and the bucket 2 are turned in that way, gravel or the like is excavated (scooped), loaded or dumped.
To assure that a next gravel scooping operation is performed at a high efficiency after gravel is loaded on a dump truck or dumped in a hopper by operating a shovel loader or the like working machine, it is required that during rearward movement of the vehicle, the booms 1 are lowered while correcting an angle of the bucket 2 from the downward attitude so as to allow the bottom surface 2a of the bucket 2 to extend horizontally (representing turning movement of the bucket 2 to the tilt side). To meet this requirement, an operator is required to visually confirm rearward movement of the vehicle as well as operation in the front area so as to allow the bottom surface 2a of the bucket 2 to horizontally extend on the ground surface, as represented by solid lines in FIG. 9. Accordingly, he is required to perform a steering operation by turning a handle as well as a lever actuation for turning the bucket 2 to the tilt side or stopping it. However, to perform these operations, a highly skilled technique is required. Further, since such operation for causing the bottom surface 2a of the bucket 2 to extend horizontally is manually performed by his visual confirmation, a scooping operation to be performed during a next cycle is accomplished at a low efficiency.
To solve the foregoing problem, a bucket leveler mechanism has been heretofore used. The bucket leveler mechanism essentially comprises a lever detent mechanism for immovably holding a bucket actuating lever at a full stroke position on the tilt side, a solenoid for releasing a lever detent in the lever detent mechanism from the immovable state and permitting the bucket actuating lever to be restored from the full stroke position to a neutral position and a proximity switch LS for detecting that the bucket cylinder 4 expands to a predetermined cylinder length with which the bottom surface 2a of the bucket 2 extends horizontally (see Fig. 10).
With such bucket leveler mechanism, when the bucket actuating lever is actuated to the full stroke position on the tilt side during rearward movement of the vehicle after gravel is loaded or dumped, it is immovably held by the lever detent mechanism, whereby the bucket 2 automatically continues to turn to the tilt side from the position where it assumes a downward attitude, even though an operator's hand is released from the bucket actuating lever. When the bucket cylinder 4 expands to a predetermined cylinder length during turning movement of the bucket 2 and thereby the proximity switch LS is actuated, this cylinder length is detected by the proximity switch LS which in turn outputs a detection signal to activate the solenoid. Consequently, the bucket actuating lever which has been immovably held at the full stroke position on the tilt side is automatically restored to the neutral position, whereby turning movement of the bucket to the tilt side is interrupted with the result that the bucket 2 is automatically stopped at a predetermined angle which is determined such that the bottom surface 2a of the bucket 2 extends horizontally. With such bucket leveler mechanism, an operator can concentrate his attention on a lowering operation of the booms 1 as well as a steering operation for the vehicle. In addition, he can concentrate his visual confirmation on rearward movement of the vehicle, resulting in an increased operational efficiency and an improved safety being assured.
With respect to the conventional bucket leveler mechanism as constructed in the above-described manner, however, since arrangement of the proximity switch LS is made such that the bottom surface 2a of the bucket 2 extends horizontally when the booms 1 are lowered to the predetermined position where the bottom surface 2a of the bucket 2 comes in contact with the ground surface, it has been found that a working machine such as a shovel loader or the like including a link mechanism comprising booms 1 and a bucket 2 fails to operate such that the bottom surface 2a of the bucket 2 extends horizontally in response to actuation of the bucket leveler mechanism, when the booms 1 are held at a position other than the predetermined lowered position where the bottom surface 2a of the bucket 2 comes in contact with the ground surface.
Accordingly, while the conventional bucket leveler mechanism is employed for the vehicle, there arise the following problems, particularly when the bucket 2 is raised up to an elevated position above the ground surface, as represented by two-dot chain lines in FIG. 9.
(1) When an operation for uniformly leveling the upper surface of gravel or the like material (hereinafter referred to as a leveling operation) is performed after a dump truck is fully loaded with gravel or the like material using a shovel loader or the like working machine, the bottom surface of the bucket does not extend horizontally while the bucket is held immovable with the conventional bucket leveler mechanism, because the bucket is normally maintained at a high position during the leveling operation. Thus, an operator is required to visually perform a correcting operation for tilting the bucket to a horizontal attitude.
(2) When a loading/unloading operation is performed using a fork FK as shown in FIG. 11 in place of the bucket, it is required that an edge of the fork FK is horizontally oriented without fail prior to loading of a cargo on the fork FK. However, when the cargo is placed on the fork FK held at a high position using the conventional bucket leveler mechanism, the fork edge fails to extend horizontally like the preceding case where the bucket is used. Therefore, he is required to visually performing a correcting operation in the same manner as mentioned above. Thereafter, as the fork FK having the cargo loaded thereon is lowered to the ground surface, the fork edge is inclined downward (forward) due to characteristics of the link mechanism and this gives rise to a danger that the cargo falls down. Accordingly, when the conventional bucket leveler mechanism is employed for the vehicle, he is required to actuate it during lowering movement of the fork so as to allow the fork edge to maintain its horizontal attitude throughout the lowering movement of the fork.
Since the conventional bucket leveler mechanism is so constructed that the bucket can keep its excavating/loading attitude only when it is held at a position in the proximity of the ground surface, an angle of the bottom surface of the bucket varies as a height of the bucket varies. Thus, the conventional bucket leveler mechanism has significant problems that a loading operation to be performed using a bucket, fork or the like means is very troublesome for an operator, he becomes tired and the loading operation is performed at a low efficiency, because he is required to change an angle of the bucket while visually monitoring the loading operation or he is required to change an angle of the fork in the course of raising/lowering of the booms.
The present invention has been made with the foregoing background in mind and its object resides in providing an apparatus for maintaining the attitude of a bucket carried by a loading/unloading vehicle which assures that the bucket can be held at a certain preset angle irrespective of how far a height of booms is varied.